Conventional personal care products including cosmetic compositions that are topically applied to the skin and hair often contain ester oils. Incorporation of ester oils into these products provides benefits such as improvement of the softness and smoothness of the skin and conditioning effects to the hair (for example, by reducing the force required to comb wet or dry hair). When incorporated into personal care products, ester oils tend to contribute substantiveness, e.g, they adhere to the substrate (hair shaft or stratum corneum) due to their polarity and they additionally provide the aforementioned benefits without contributing negative aesthetics such as oiliness and greasiness.
Shine, gloss, and clarity are attributes that are often highly desired in personal care products, especially those that are applied to the skin, hair and nails. For example, shine and gloss are especially important in products such as lipsticks, lip glosses (clear or pigmented), and other cosmetics used to impart color to the skin (“decorative cosmetics”). Clarity is a particularly desirable characteristic in an antiperspirant composition. For hair care products, shine and gloss are highly desired, mostly in applications such as hair tonics, mousses, setting lotions, dressings, and conditioning compositions.
Personal care products, such as the cosmetic compositions described above, are typically applied as a composition that has the form of an emulsion, a solution, a suspension, a gel, and/or a solid. After application, a layer consisting of a mixture of the non-volatile, non-penetrating ingredients is left on the surface of the substrate. It is this layer that provides the desired color, aesthetic, beautification, and/or therapeutic benefit.
The attributes of shine and gloss are directly related to the smoothness of the surface layer after application and to the aggregate reflectivity of the components that comprise the layer. The Fresnel law of diffraction states that the intensity of a reflected beam of light emanating from air that is incident on a surface of a material is a function of: (1) the angle of incidence, (2) the polarization of the incident beam, and (3) the index of refraction of the material. Reflectivity (“R”) is defined as the ratio of the intensity of the reflected beam (“Ir”) to the intensity of the incident beam (“Ii”). By application of the Fresnel law, the equation relating R to the refractive index of the surface material (“n”) is given by:R=Ir/Ii=(n−1)2/(n+1)2 Thus, the higher the refractive index of the layer of personal care product, the greater its reflectivity.
In the case of a personal care product, the material that comprises the surface layer is a mixture of non-volatile, non-penetrating ingredients that may not be miscible in one another. Various equations have been developed using a combination of the electromagnetic theory of light and empirical methods to predict the refractive index of mixtures. The most widely used mixing rule for the prediction of the refractive index of binary systems is that of Lorentz-Lorenz. The Lorentz-Lorenz equation states that the square of the mixture refractive index minus one divided by the square of the mixture refractive index plus two is equivalent to the sum of the volume fraction of the individual components, times their respective refractive index squared minus one divided by their square plus two respectively. This relation can be extended to include ternary, quaternary, and higher order mixtures.
In formulating a personal care product one may estimate the compositions of the immiscible phases in the surface layer to calculate their respective refractive indices. With this information, one may implement a formulation strategy to achieve clarity in the final formulation by matching the refractive indices of all phases within the surface layer. The lesser the difference between/among each of the refractive indices, the greater the clarity, shine and/or gloss of the final product.
In personal care products where clarity, shine and or/gloss may be desirable, the relatively low refractive indices of most ester oils makes it difficult to achieve these properties, as the formulations also contain primarily compounds of lower refractive indices, such as castor oil, high viscosity silicones (e.g., 100 centipoise to 1000 centipoise), film formers, and/or high viscosity esters. Yet, elimination of these compounds is undesirable as well, because of the advantageous properties they contribute to the formulation.
Thus, there is a need for liquid, light viscosity, non-irritating, non-penetrating esters for use in personal care compositions with high refractive indices, such that improved shine, gloss and clarity can be obtained. Moreover, personal care compositions with these esters incorporated into their formulations are also desirable.